Allyl chloride is a useful commercial chemical especially as an intermediate in the manufacture of epichlorohydrin and glycerine. The principal commercial method for making allyl chloride is the thermal chlorinination of propylene. The problem in this chlorination essentially is to mix propylene and chlorine at the elevated temperature rapidly enough to obtain reasonable yields of allyl chloride which, with good mixing and a high ratio of propylene to chlorine, may reach 80 percent. The principal remaining organic constituents of the reaction product are 1,2-dichloropropane (propylene dichloride) from the addition of chlorine to propylene, and 1,3-dichloropropene from the further chlorination of allyl chloride together with smaller amounts of mono-, di-, and trichlorinated C.sub.3 's. Some of these compounds are useful per se, e.g., 1,3-dichloropropenes are widely used as nematocides and soil fumigants. Some of the by-products, however, must be removed and disposed of since they are not useful of themselves. The useful method of disposal is to use these by-products as a feed for the preparation of more highly chlorinated hydrocarbons or by burning them and recovering heat and the chlorine values as HCl. It would be desirable if the amounts of by-products could be reduced or the kind of by-products be controlled so that a better yield of the desired products is obtained and/or the disposition of by-products could be more easily and economically achieved.
Thermal cracking of saturated compounds to produce unsaturated compounds is a highly developed art. Dehydrogenations and dehydrohalogenations to give unsaturated compounds and hydrogen or a hydrohalide are well known as commercially useful processes. The thermal cracking of ethylene dichloride to give vinyl chloride and of ethane to give ethylene are two examples of many which could be cited. Although catalysts are sometimes employed in such reactions, the use of high temperatures alone is usually sufficient and most frequently employed. Chlorinated hydrocarbons which contain unsaturation also undergo dehydrochlorination under these conditions. The result can be the formation of triple bonded compounds which are highly unstable and which tend to decompose rapidly to form carbonaceous materials which plug process equipment and prevent efficient operation.
It has now been discovered that a combination of direct chlorination and cracking steps can be used in order to render these by-products more ecologically acceptable and/or useful in recycling to the process in order to improve the overall yield to allyl chloride or other desirable products and avoid carbon formation.
Chlorination of unsaturated chlorohydrocarbons can result in either addition to the double bond or hydrogen substitution depending on the reaction conditions. The substitution reaction is also possible with saturated chlorohydrocarbons. Since, for the purposes of the present invention, the addition reaction generally results in the formation of more desirable materials, it is advantageous to operate the chlorination step using reaction conditions to favor the addition reaction and/or minimize substitution. For example, the addition reaction is favored by chlorination at low temperatures and in the presence of catalysts such as FeCl.sub.3, while substitution is favored by elevated temperatures and actinic radiation.
In the common commercial method for the manufacture of allyl chloride by the substitution chlorination of propylene the preferred temperature is approximately 500.degree. C. At these temperatures carbonaceous materials are invariably formed. Various schemes have been advanced in the past to reduce carbon formation, but they have met with very limited success. These schemes have generally involved the use of reactor design or diluents to prevent "hot spots" in the reactor.
The carbon, once formed, causes numerous problems. For example, it eventually plugs the thermal chlorination reactor, which must then be shut down and cleaned. Carbon build-up on the walls of the reactor can lead to the production of increased amounts of undesirable by-products. Also, extremely small particles of carbonaceous material remain in the reactor effluent and are almost impossible to remove. These small particles tend to enter the plant finishing system where they cause many problems. It would be extremely advantageous if carbon formation in the thermal chlorination reactor could be prevented or at least greatly reduced.